@Ken iTunes does create a backup, but the only use for that backup is to restore it back to your device. In order to actually get the image and video files you'll have to transfer with iPhoto, Image Capture, or another photo management application.
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Kyle CroninDec 4 '10 at 6:53

You should be using iPhoto (or other digital camera software) to copy these photos onto your Mac, but just for sake of completeness, they are included in the backups that iTunes makes.

Those backups can be found in ~/Library/Application Support/Mobile Sync/Backup, but you need a special program (iPhone Backup Extractor) to be able to read them (and that only works if they are unencrypted).

You can find your camera roll after extracting the area called "iOS Files" in /Media/DCIM.